1. Field of the Invention
This invention relates to a shading correction method and device for correcting shading distortion resulting from nonuniformity in sensitivity of an image reading system including an image sensor and other possible causes. More particularly, this invention relates to a method and device capable of effectively and reliably making shading correction in the image reading system to a high degree with a simple processing circuit.
2. Description of the Prior Art
In digital image reading systems such as an image reader, digital copying machine and facsimile, a given original image such as characters and patterns to be processed is illuminated with a light source and optically read by photosensitive elements constituting an image sensor, thereby to induce electric image signals. Due to ununiform illumination of the light source and nonuniformity in sensitivity of the image sensor, nonuniformity in density appears in an image reproduced resultantly, which disagrees with the original image. This phenomenon is generally called "shading distortion" and inevitably takes place in the image reading system of this type.
For instance, the light source for illuminating the original image is irregularly degraded gradually in illumination intensity during the course of prolonged service. Also, the sensitivity of the image sensor is irregularly reduced during prolonged use. In general, the shading distortion due to such causes appears in the form of dark stripes extending in the sub-scanning direction in the reproduced image finally obtained.
In order to avoid appearance of the shading in the reproduced image, it is required to effect previous shading correction by reading a white standard surface prior to read-scanning operation for an objective image designated to be processed.
To be more specific, the standard surface s is partitioned into m in the main-scanning (row) direction and into n in the sub-scanning (line) direction to determine an mxn matrix in soft and prescribe pixels (image elements) El . . . Ei . . . Em in each line, as illustrated in FIG. 1. The pixels are sequentially read out by line with the image sensor 1 having m-photosensitive elements arranged in an array. The standard surface s is scanned by relatively moving the image sensor 1 in the sub-scanning direction to read out image data in n-scanning line Ll . . . Lj . . . Ln by line.
Since the standard surface s is white, all image values outputted from the image sensor which represent the brightness of the pixels should be maximum in theory, but in reality the output signals So (o=l . . . j . . . m) from the photosensitive elements are not constant as schematically shown by the graph in FIG. 1 due to the causes noted above. In the drawing, remarkable electric drops of the output signals Si are found in common in i-row, which are considered as the shading distortion. When an electric drop occurs locally like the pixel Ek in the row k of the line Lj in the drawing, it may be possibly caused due to a dark spot on the white standard surface or electrical noise such as electrical disturbance. In any case, these undesirable matters must be eliminated to reproduce the image with accuracy.
There have been conventionally proposed various methods for effecting shading correction, as follows:
U.S. Pat. No. 4,003,023 [H. SASAKI ET AL.]
U.S. Pat. No. 4,523,229 [Y. KANMOTO]
U.S. Pat. No. 4,691,365 [N. NAGASHIMA]
U.S. Pat. No. 4,829,379 [T. TAKAKI]
All these prior art are related by and large to a shading correction technique in which low level signals of image data produced by previously reading out a white standard surface by line are substituted with a reference "mean value" obtained by averaging the image data read out in one scanning line. The low level signal St in the sensitive characteristic pattern FP is raised to the mean value Vm as schematically shown by a dotted line in FIG. 1 by way of example.
The conventional shading correction method in which the low level signals are substituted with the mean value can be carried out by use of an easy arithmetic algorithm and a simple processing circuit, but accurate shading correction cannot be expected. This is because the mean value obtained by averaging the entire sensitivity of photosensitive elements of an image sensor is lowered by aging as a matter of course, only to incur increase of shading distortion. Furthermore, the conventional shading correction method is affected quite sensitively by the conditions of the white standard surface used in a preliminary read-scanning process. For instance, undesirable factors on the white standard surface such as dark spots or image noises are regarded as an objective to be corrected, consequently to frustrate the shading correction.
Another prior art, U.S. Pat. No. 5,260,809 [K. TACHIKAWA; Nisca's U.S. Patent] (corresponding to Japanese Patent Application Public Disclosure No. HEI 2-65463) discloses a shading compensating method in which shading correction in an image reading system is made by the steps of determining a comparative value by previously scanning white and black standard surfaces, and multiplying shading correction value represented as the ratio of actual image values obtained by scanning a given objective image to the comparative values by coefficient representing gradation of a resultant image to be reproduced.
Thus, the shading compensating method performs a high level aritbanetic operation of multiplying the gradation of the reproduced image by the shading correction values given by the ratio of the actual image values to the comparative values of the white and black standard values, but it calls for a relatively complicated processing circuit for effecting the arithmetic algorithm.
A method of correcting a local noise on a white standard surface used in a preliminary scanning process is proposed in Japanese Patent Appln. Public Disclosure No. SHO 63-155869. In this prior art method, the local noise appearing at a specific pixel in a specific scanning row on the white standard surface is corrected by substituting an image value of the specific pixel with a median value selected from image values of all pixels in the specific scanning row. To effect the correction of the local noises on the standard surface with a high degree of accuracy by the prior art method, all the image signals detected from all the pixels prescribed on the standard surface must be stored in memories such as latch memories until the scanning of all of the pixels on the standard surface is finished. Although such a method essentially calls for enormous memories, the proposed method attempts to reduce the number of memories by thinning down image data read out from an objective original image to substantially scan the original image roughly, and furthermore, the median value extracted by the proposed method leaves change in the sub-scanning direction out of consideration, resulting in decrease in accuracy of image reading.
Assuming that the local noise correction method noted above is simply applied to shading correction, it requires too many memories to be practicable.